1. Field of the Invention
The present invention relates to an electric power steering apparatus which provides power assist of an electric motor directly to a steering system so as to reduce necessary steering power to be applied by the driver.
2. Description of the Related Art
FIG. 1 of the accompanying drawings diagrammatically shows the general construction of an electric power steering apparatus of the type concerned.
The electric power steering apparatus 1 includes an electric motor 10 disposed in a steering system of the vehicle, and a controlling unit or controller 20 for controlling power assist of the electric motor 10 so as to reduce a manual steering effort or power required of the driver.
A steering wheel 2 of the vehicle is firmly connected to one end of a steering shaft 3, the opposite end of the steering shaft 3 being operatively connected through a pair of universal joints 4a, 4b to a pinion gear 6 of a rack-and-pinion mechanism 5. The rack-and-pinion mechanism 5 includes a rack shaft 7 having formed on its outside surface a row of rack teeth 7a meshing with the pinion gear 6. By means of the rack-and-pinion mechanism 5 formed jointly by the pinion gear 6 and the rack teeth 7a, rotary motion of the pinion gear 6 is translated into axial reciprocating motion of the rack shaft 7. The rack shaft 7 is connected at its opposite ends to steerable front wheels 9, 9 via a pair of tie rods 8, 8, respectively.
With this arrangement, when the steering wheel 2 is steered or turned in one direction, the front wheels 9 are caused to pivot or swing in the same direction via the rack-and-pinion mechanism 5 and the tie rods 8, thereby changing the direction of motion of the vehicle.
In order to reduce necessary steering power to be applied by the driver, the electric motor 10 for providing a steering assist torque (power assist) is disposed coaxially with the rack shaft 7 in such a manner that rotational output power of the electric motor 10 is converted via a ball screw mechanism 11 into an axial thrusting force acting on the rack shaft 7. The ball screw mechanism 11 is composed of a nut 12 connected to a rotor of the electric motor 10 for co-rotation therewith, and a screw shaft 7b formed on a part of the rack shaft 7 and threaded with the nut 12. With this arrangement, a rotational force of the nut 12 is converted via the screw shaft 7b into an axial thrusting force of the rack shaft 7. Since the steering assist torque generated by the electric motor 10 is thus converted into the axial thrusting force of the rack shaft 7, a manual steering force required of the driver to steer the front wheels 9 can be reduced.
A steering torque sensor 18 detects a manual steering torque T acting on the pinion gear 6 and generates a steering torque signal 18a indicative of the detected steering torque T. The steering torque signal 18a is supplied to the controller 20. The controller 20 generates a motor drive signal 20a on the basis of the torque signal 18a, so as to control output power of the electric motor 10 according to the motor drive signal 20a.
FIG. 2 is a block diagram showing a typical known example of the controller 20. As shown in this figure, the controller 20 generally comprises an assist torque control section 30 and a motor drive section 40. The assist torque control section 30 determines a target assist torque based on the torque signal 18a from the steering torque sensor 18 and generates a target assist torque signal 30a. The motor drive section 40 generates a motor drive signal 20a based on the target assist torque signal 30a.
The assist torque control section 30 includes a steering torque compensator 31, a first target assist torque determiner 32, a steering torque differentiator 33, a second target assist torque determiner 34, and an adding means or adder 35.
The steering torque compensator 31 undertakes compensation of the frequency characteristics of the steering torque signal 18a and generates a compensated steering torque signal 31a. To this end, the steering torque compensator 31 has a frequency characteristic so set as to stabilize operation of the steering system.
The target assist torque determiner 32 generates a first assist torque signal 32a based on the compensated steering torque signal 31a. The first assist torque signal 32a is a signal corresponding to a drive current (including the polarity thereof) of the electric motor 10 which is required for the electric motor 10 to generate a first target assist torque. The first target assist torque signal 32a may be a signal corresponding to a drive voltage of the electric motor 10 which is required for the electric motor 10 to generate the first target assist torque.
The first target assist torque determiner 32 has a steering torque vs. first target assist torque conversion table which is prepared to output a value of the first target assist torque with respect to the input of a value of the compensated steering torque. The first target assist torque determiner 32 is designed to set the first target assist torque value to be zero when the absolute value of the compensated steering torque value is smaller than a preset dead zone threshold. Alternatively, when the compensated steering torque value is greater than the preset dead zone threshold, the first target assist torque determiner 32 operates to output a first target assist torque value proportional to the compensated steering torque value. The first target assist torque determiner 32 controls the first target assist torque value so as not to exceed a preset first target assist torque upper limit even when the compensated steering torque value becomes large.
The steering torque differentiator 33 determines the amount of change in the steering torque signal 18a per unit time and outputs the determined change of the steering torque signal 18a as a differential torque signal 33a.
The second target assist torque determiner 34 outputs a second target assist torque signal 34a based on the differential torque signal 33a. The second target assist torque signal 34a is a signal corresponding to a drive current (including the polarity thereof) of the electric motor 10 which is required for the electric motor 10 to generate the second target assist torque. The second target assist torque signal 34a may be a signal corresponding to a drive voltage of the electric motor 10 which is required for the electric motor 10 to generate the second target assist torque.
The second target assist torque determiner 34 has a differential torque vs. second target assist torque conversion table which is prepared to output a value of the second target assist torque with respect to the input of a differential torque value. The second target assist torque determiner 34 is designed to set the second target assist torque value to be zero when the absolute value of the differential torque value is smaller than a preset dead zone threshold. The dead zone threshold of the second target assist torque determiner 34 is set to be smaller than the dead zone threshold of the first target assist torque determiner 32. When the differential torque value is greater than the preset dead zone threshold, the second target assist torque determiner 34 operates to output a second target assist torque value proportional to the differential torque value. The second target assist torque determiner 34 controls the second target assist torque value so as not to exceed a preset second target assist torque upper limit even when the compensated steering torque value becomes large.
The adder 35 operates to add together the first target assist torque signal 32a and the second target assist torque 32a and output the result of the adding operation as the aforesaid target assist torque signal 30a.
The motor drive section 40 is comprised of a deviation calculating circuit 41, a PID (proportional-integral-derivative control) circuit 42, and a drive circuit 43.
The electric motor 10 is equipped with a current detector (not shown) which detects a current being actually supplied to the electric motor 10 and outputs a drive current signal 10a corresponding to the detected motor current. The drive current signal 10a is fed back to the deviation calculating circuit 41 of the motor drive section 40.
The deviation calculating circuit 41 determines a deviation between the target assist torque signal 30a (i.e., a signal corresponding to a current to be supplied to the electric motor 10 to generate the target assist torque) and the drive current signal 10a, and it outputs a deviation signal 41a.
The PID circuit 42 is designed to perform specific computing operations, such as proportion, integration and differentiation on the basis of the deviation signal 41a and to output a PID control output signal 42a to the drive circuit 43.
The drive circuit 43 generates, on the basis on the PID control output signal 42a, a motor drive signal 20a and provides it to the electric motor 10 so as to supply a drive current (or a drive voltage) to the electric motor 10.
The conventional controller 20 of the foregoing construction operates to set both a first target assist torque in accordance with a steering force exerted by the driver and detected by the steering torque sensor 18, and a second target assist torque in accordance with the change of the steering force, and to control, on the basis of a target assist torque obtained by adding together the first target assist torque and the second target assist torque, an assist torque to be provided from the electric motor 10.
In the conventional controller 20, the steering torque compensator 31 is disposed immediately upstream of the first target assist torque determiner 32 so as to stabilize the operation of the steering system. Rotational power of the electric motor 10 is fed back to the steering torque sensor 18 via the ball screw mechanism 11 and the rack shaft 7. The electric motor 10, the ball screw mechanism 11, the rack shaft 7, the steering torque sensor 18, the steering torque compensator 31, the first target assist torque determiner 32, the adder 35 and motor driving section 40 are operatively connected together in the order named and jointly form a first closed control loop. In the first closed control loop, the steering torque compensator 31 undertakes compensation of the frequency characteristics in an effort to stabilize control operation of the first closed control loop.
However, when the compensated steering torque signal 31a supplied as an output from the steering torque compensator 31 has such a small level which is included in a dead zone of the first target assist torque determiner 32 (namely, smaller than the dead zone threshold), the first target assist torque signal 32a supplied from the first target assist torque determiner 32 is fixed to the value of 0 (zero). In this condition, the first closed control loop (starting from the steering torque sensor 18, then passing successively through the steering torque compensator 31, the first target assist torque determiner 32 and the motor drive section 40, and finally returning to the steering torque sensor 18), i.e., a closed loop containing the first target assist torque determiner 32 cannot undertake a prescribed closed loop control action. In other words, when the output level of the steering torque compensator 31 is in the dead zone of the first target assist torque determiner 32, the frequency characteristics compensation performed by the steering torque compensator 31 becomes totally ineffective.
There is another or a second closed loop in the controller 20, which loop starts from the steering torque sensor 18, then passes successively through the steering torque differentiator 33, the second target assist torque determiner 34, the adder 35, the motor driving section 40 and the electric motor 10, and finally returns to the steering torque sensor 18 (i.e., a closed loop including the second target assist torque determiner 34). Even when the absolute value of a steering torque value is small to such an extent as to belong to the dead zone of the first target assist torque determiner 32, if the amount of a change in the steering torque per unit time (represented by a differential torque signal 33a supplied from the steering torque differentiator 33) exceeds the dead zone threshold of the second target assist torque determiner 34, an assist torque based on the change of the steering torque will be supplied. However, the second closed loop including the second target assist torque determiner 34 does not have any means for adjusting or compensating the frequency characteristics to stabilize the control operation of the second closed loop. Accordingly, if a control operation of the controller 20 based only on the second closed loop including the second target assist torque determiner 34 is performed, the control operation tends to become unstable.
In the conventional controller 20, although an attempt was made to increase the proportion of the second target assist torque to the first target assist torque so as to provide a light feel to steering maneuvers, the result was unsatisfactory in that when steering is taken on a road having a low tire ground-contact coefficency (such as a snow-clad road) and hence requires only a small manual steering force (i.e., when the level of an output from the steering torque compensator 31 is in the dead zone of the first target assist torque determiner 32), partly due to a small frictional force acting between the road surface and the tires, and partly due to an insufficient control-loop stabilizing operation of the steering torque compensator 31, the steering system becomes unstable and sometimes causes unpleasant vibrations of the steering wheel 2. Thus, the conventional controller 20 has a limited capability of increasing the proportion of the second target assist torque to the first target assist torque in an effect to improve the comfortableness of steering maneuvers without deteriorating the stability of control operation of the steering system.